Molecular Suppression of PCBM Dimerization in Inverted Perovskite Solar Cells

Inverted perovskite solar cells require robust electron‐transport layers (ETLs) to fully realize their potential in both efficiency and operational stability. The widely used ETL material phenyl‐C ₆₁ ‐butyric acid methyl ester (PCBM) is intrinsically vulnerable to photo‐induced [2 + 2] cycloaddition, which drives dimerization, aggregation, and morphological instability, ultimately degrading charge transport. Here we show that incorporating tetraphenylethylene (TPE) effectively mitigates these limitations. The propeller‐shaped, nonplanar TPE molecules impose strong steric hindrance that prevents the close molecular approach required for PCBM photodimerization, while their extended π system establishes stabilizing π–π and CH···π interactions that anchor PCBM within a uniform, aggregation‐resistant matrix. This molecular‐level regulation yields PCBM:TPE ETLs with markedly improved film uniformity and electron mobility. Inverted devices employing this engineered ETL achieve a power conversion efficiency of 26.6% with a fill factor of 85.1%, together with substantially enhanced operational stability under ISOS‐L‐3 conditions (65°C, 50% relative humidity, and ambient air). These results establish a generalizable molecular strategy to suppress deleterious fullerene photochemistry and stabilize charge‐transport layers in high‐performance perovskite photovoltaics.